Collect duplicate fields using derive attribute

[Plecra]

This continues #690. @dtolnay recommended using a Deserializer adapter as a solution, but I'm not sure how it helps. I don't think there's any Deserialize method that could be used for this behaviour.

#[derive(Deserialize)]
struct Cores {
    #[serde(duplicates)]
    core: Vec<String>,
}

Here's an possible implementation on the playground

[nickbabcock]

Yeah I ended up implementing this with my own custom derive macro for structs (based on your gist -- thank you again for showing me)

Some implementation detail differences in behavior when a field is decorated with #[serde(duplicated)] compared to a regular field:

• Initialized with Vec::new() (ie: not initialized to an std::option::Option::None)
• When the __Field is encountered, always blindly push the next value:

(#name).push(::serde::de::MapAccess::next_value::<#farg>(&mut __map)?);

• No need to have a step to extract field from an Option or check if there is a default fallback for said field.

There are some significant downsides to implementing one's own custom derive macro -- I haven't implemented many of serde's field / container attributes. I've also only gotten duplicated to work for the Vec containers. So if there does exist a Deserializer adapter for a solution, I'd love to see it!

[dtolnay]

I still don't feel this is something that needs to be built into the serde crate. Thanks anyway for the suggestion!